This invention relates to detecting and monitoring underground contamination using a neutron activation gauge, sometimes referred to as a neutron backscatter gauge. Further, this invention relates to monitoring the biodegradation of underground contaminants and optionally performing active remediation.
A neutron backscatter gauge utilizes a source of fast neutrons and a detector for slow neutrons. The presence of hydrogen-bearing material, such as an organic liquid or water, is detected by a change in the number of slow neutrons counted in the detector. The number of backscattered slow neutrons is directly related to the number of hydrogen atoms present for slowing the fast neutrons from the source.
A representative neutron backscatter gauge emits fast neutrons from a source comprising americium-241 and beryllium, which require on average about 19 collisions with hydrogen atoms for thermalization of an energy level detectable by a helium-3 gas detector tube. The output of the gauge is a number of "counts" representative of the number of slow neutrons detected and hence the number of hydrogen atoms in the test volume the detector can survey. There can be as much as 10% scatter in the neutron gauge output. This can be reduced by operating the gauge for a longer period which tends to average scatter in the results. Fifteen seconds for a measurement is a reasonable period.
In the prior art, neutron detectors have been used in well logging. Examples of these applications are seen in Mills U.S. Pat. No. 4,268,749. It has also been proposed to measure leakage from pipe lines by passing a neutron backscatter gauge through the pipe line. Such a technique is described in Scherbatskoy U.S. Pat. No. 3,060,315.
There is a need for detecting contamination, for example due to leaks from underground tanks and pipes, particularly tanks that were installed decades ago, using a nondestructive and non-invasive method that is at the same time economical and efficient. Once these leaks are detected, there is a further need to monitor the biodegradation of the contaminants and optionally provide active remediation when needed.